cgen: fix print interface when arg0 is function call (fix #26184) (#26188)

Author: kbkpbot

vlib/v/gen/c/fn.v

@@ -2143,7 +2143,9 @@ fn (mut g Gen) fn_call(node ast.CallExpr) {
 		}
 	}
 	if !print_auto_str {
-		if is_print {
+		if is_print && node.args[0].expr !is ast.CallExpr {
+			// only need for `println(err)`
+			// not need for `println(err.msg())`
 			g.inside_interface_deref = true
 			defer(fn) {
 				g.inside_interface_deref = false

vlib/v/tests/interfaces/interface_err_msg_print_test.v

@@ -0,0 +1,27 @@
+struct MyError {
+	code  int
+	state string
+}
+
+pub fn (e MyError) msg() string {
+	return 'something went wrong'
+}
+
+pub fn (e MyError) code() int {
+	return e.code
+}
+
+fn foo() ! {
+	return MyError{}
+}
+
+fn test_interface_err_msg_print() {
+	foo() or {
+		if err is MyError {
+			eprintln(err.msg())
+			exit(err.code())
+		} else {
+			panic(err)
+		}
+	}
+}